Integration tests fail for python 3.9 and 3.10

[vhaasteren]

The codev integration tests fail for python 3.9 and 3.10, with the message

FAILED tests/test_vector_parameter.py::TestVectorParameter::test_vector_parameter_like - TypeError: arrays to stack must be passed as a "sequence" type such as list or tuple.

This is due to the line

xs = np.hstack(p.sample() for p in pta.params)

in test_vector_parameters_like.

PR #352 fixes this issue. Until merged, no new PRs will pass the integration tests.